One task facing radiographers using conventional radiographic techniques (e.g., x-ray film) is to control the amount of exposure of the photographic film or plate. Specifically, during an examination, the total amount of x-rays passing through the patient's body part and striking the film or plate determines whether the film or plate is under, over, or properly exposed.
Typically, one of two techniques is used for controlling the exposure of an x-ray film or plate to an x-ray field. In the first, an ionization chamber is placed between the body part and the film. As the x-rays exit the body, they pass through the chamber before striking the film. The passage of the x-rays through the gas in the chamber produces an electrical charge proportional to the sum of the x-ray exposure. Once a level of charge indicative of the proper amount of exposure has been reached, the exposure is terminated. This technique, however, measures the exposure over the totality of the chamber and does not allow for localized exposure measurements. Thus, if a precise image of an area significantly smaller than that of the chamber is required, multiple images may have to be taken in order to correctly expose the localized area.
In the second commonly used technique for controlling levels of x-ray exposure, a photo cell is used to measure the accumulated x-ray exposure over a portion of an x-ray film or plate. Since only a portion is measured, accurate imaging of localized body parts is much easier to achieve than by use of an ionization chamber. After passing through the patient and the film, x-rays strike the photo cell where they produce an electrical charge in proportion to their intensity. Since, however, a photo cell is limited in the area it may measure, the body part in question must be carefully positioned between the source of the x-ray field and the cell. If the body part does not completely block the photo cell from raw x-rays, the exposure will terminate prematurely. As a result, it is very difficult to properly capture x-ray images of body parts positioned near the edge of the patient's body using the photo cell technique. In addition, since the photo cell is a separate unit from that of the x-ray plate, the cell must be properly aligned and positioned before it can produce a meaningful measurement.
Recently, digital radiography systems have begun to replace conventional x-ray systems. Digital radiography systems provide high quality radiographs by capturing x-ray images with a sensor plate having a matrix of silicon detectors. The x-ray images can be transmitted to a diagnostic viewer or any other output device, or to any other location via, for example, an Ethernet interface.
The sensor plate provides several advantages over conventional x-ray film. For example, unlike conventional x-ray film, digital images can be produced and viewed within a few seconds of x-ray exposure. Moreover, the sensor plate used in digital systems can capture most patient imaging areas with high resolution (e.g., 160.times.160 microns pixel size, with 4096 gray scale (12 bit) contrast). The sensor plate also covers a larger dynamic range than conventional x-ray film.
Like conventional x-ray film, digital radiography systems require techniques for controlling the level of x-ray exposure experienced by the digital sensor plate. Currently, digital radiography must make do with the conventional x-ray system techniques and the associated problems discussed above. It is desirable therefore to have a system and method for accurately controlling a localized level of exposure, without having to precisely position the patient and the measuring device.